Ion-molecule reaction studies of hydroxyl cation and ionized water with ethylene

Rate coefficients and product branching ratios for the ion-molecule reactio ns of the hydroxyl cation, ionized water, and their deuterated analogues wi th ethylene have been determined using a selected ion flow tube (SIFT) at r oom temperature and in 0.5 Torr of helium buffer gas. In all cases, reactio ns proceed at or near the collision rate, The major product is always charg e transfer: 79% for L2O.+ and 66% for LO+ and does not depend on the isotop ic form of hydrogen present (L = H or D). For the L2O.+ reactions, the rema ining 21% of products are from proton or deuteron transfer, with no evidenc e of an isotope effect on this step even in the HOD.+ reaction. The greater exothermicity of the initial charge transfer in the LO+ reaction is reveal ed by the observation of additional product channels, forming the vinyl cat ion and protonated carbon monoxide. Multistep mechanisms that proceed throu gh rate-determining charge-transfer, followed by a product-determining step , are postulated to explain these observations.